Ha ha, lol. Do you put the earphones in or your child? Make sure the volume is low enough for their hearing, and then just have them available without being the one to put them in. A child learns through mimicking their parents so have a separate set for yourself and pay your child no attention. Curiosity (a natural human trait) will get the better of them and soon they will follow suit.

Introduce it at a safe time away from the hustle and bustle and maybe have some nursery rhymes (Zoodles are a great app for this) on preset.
Then when your at the store just reintroduce it again. Your child will remember the fun of the previous times and be joyfully humming a baa baa black sheep beat in no time at all.